Two or more liquid components can be separated, if their volatilities are different, by the well known process of distillation. If the volatilities are widely different, simple flash distillation might give an adequately pure stream of the more volatile component or components, but otherwise the vapour phase must usually be rectified in a fractionating column. Within the fractionating column, counter-currently flowing vapour and liquid phases are intimately contacted, with the liquid reflux stream usually being provided by an external condenser which rejects heat to the surroundings.
The principal energy input to the distillation process is heat, which is applied to a boiler to drive off vapour which consists predominantly of the more volatile component or components. In many instances, such as in an absorption refrigeration cycle, it is desirable to utilise this heat as efficiently as possible; that is, to use as little heat as possible to produce the required degree of component separation. The strategy for so doing involves preheating the feed mixture with liquid and vapour from the boiler. For a flash process, this can reasonably be achieved using conventional equipment, although controlled splitting of the feed stream to contact the liquid and vapour streams separately would probably be required. If the vapour from the boiler must be rectified, however, the inconvenient step of incorporating a heat exchange surface into the fractionating column is required if heat is to be recovered from the vapour stream. In common practice, only recovery of heat from the liquid stream is undertaken and the energy efficiency of the rectification process is maximised by operating with as low a liquid reflux ratio as possible.